System and method for storing energy in a nuclear power plant

ABSTRACT

A method for storing the energy of a nuclear power plant in which the nuclear core is cooled by gases or liquid heat transfer media. The hot heat transfer liquid is stored directly in storage tanks. When needed, it is used for heating a power plant. The heat of a compressed gas heat transfer medium such as helium is stored by passing the compressed gas through tanks filled with heat-resistant solids and recovered by passing the same type of gas in a second circuit in a reverse direction. Through the hot tanks to the power plant and back. This Abstract is not intended to define the invention disclosed in the specification, nor intended to limit the scope of the invention in any way.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. application Ser. No.12/376,064, filed Feb. 2, 2009, now U.S. Pat. No. 8,724,768, which inturn is a U.S. national stage application under 35 U.S.C. 371 ofInternational Application No. PCT/US07/74647 filed Jul. 27, 2007 andentitled SYSTEM AND METHOD FOR STORING ENERGY IN A NUCLEAR POWER PLANT,which in turn claims the benefit of U.S. Provisional Patent ApplicationNo. 60/834,736, filed on Aug. 1, 2006, under 35 U.S.C. §119(e), thedisclosures of which are expressly incorporated by reference herein intheir entirety for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates a system and method of equipping specific types ofnuclear power plants with low cost storage that has a very high thermalefficiency.

The invention also relates to systems and methods for operating nuclearreactors cost-effectively at maximum capacity so that nuclear plantswill be able to compete with conventional fossil fueled power plants intheir responsiveness to load changes over a wide range.

The invention also relates to a storage system and method for ahigh-temperature gas-cooled nuclear reactor wherein the storage systemor the method has a high efficiency (over 90%) and a low cost, allowingthe nuclear reactor to always operate at maximum reactor power, whileremaining capable of varying its electrical output as does a steam powerplant.

2. Discussion of Background Information

Nuclear reactors have a large thermal inertia, which slows theirresponsiveness to variations in the demand for power from the grid.Their potential to become the major source of electricity is seriouslyaffected by this limitation. Additionally, the initial cost ofinvestment in a nuclear power plant is high; therefore, they must bebuilt to operate at full capacity as it is too costly to operate them atlow loads. Commercial nuclear reactors are kept operating full time toensure a profitable return on the original investment, therefore, mostnuclear reactors are designed for base load. Operating them at or belowhalf-capacity is not economically attractive since halving the loadnearly doubles the cost per KWh. Another limitation on theirfunctionality is that due to their thermal inertia, nuclear reactors canhave a slow transient response.

As currently designed, nuclear power plants are unable to follow thevariable demands of the grid because they are expensive to operate atintermediate loads and unsuitable for rapid load following. Because theyare used mostly for base power, the total contribution they can make tothe grid is thereby limited. Sixty percent of the demand for electricityis for variable, controllable power. At present, this need is suppliedby coal-fired steam and gas turbine power plants and to some extent byhydroelectric power. While coal-fired steam power plants can respond toload changes quickly and can operate well with a load of only 13% ofdesign capacity, they are more expensive to use for generatingelectricity during periods of partial load as they must be designed formaximum capacity.

Various energy storage devices have been proposed to solve this problem,but all of these proposals have limited efficiency (about 75%) and areexpensive. Furthermore, while storage systems have been proposed forsolar thermal power plants (see Sargent & Lundy, “Assessment ofParabolic Trough and Power Tower Solar Technology Cost and PerformanceForecasts”, SL-5641, (2002), the disclosure of which is hereby expresslyincorporated by reference in its entirety), they are based on liquidheat transfer fluids and molten salts, which may be unsuitable fornuclear reactors. The system described therein uses hundreds tothousands of sun-tracking mirrors called heliostats to reflect theincident sunlight onto the receiver. These plants are best suited forutility-scale applications in the 30- to 400-MWe ranges. In amolten-salt solar power tower, liquid salt at 290° C. (554° F.) ispumped from a “cold” storage tank through the receiver where it isheated to 565° C. (1,049° F.) and then on to a “hot” tank for storage.When power is needed from the plant, hot salt is pumped to a steamgenerating system that produces superheated steam for a conventionalRankine-cycle turbine/generator system.

Consider, for example, a high temperature nuclear reactor cooled byhelium (He) or any intermediate heat transfer medium (see Baxi, C. B.,et al.; “Evolution of the Power Conversion Unit Design of the GT-MHR”,presented at the International Congress on Advances in Nuclear PowerPlants, (2006), the website the entry for Pebble Bed Reactor onWikipedia and Penner, S, S.; Seiser, R. Schultz, K.; “Nuclear Energy forthe Future”, Presented at the Meeting of the Doctors for DisasterPreparedness, Las Vegas Nev., 16-17 Jul. 2005, the disclosures of whichare hereby expressly incorporated by reference in their entireties).

The invention solves one or more of the problems associated withconventional nuclear power plants, is simple in design, is more robust,is cheaper and lacks one or more of the disadvantages of conventionalnuclear power plants.

SUMMARY OF THE INVENTION

The invention provides for a system and method for equipping specifictypes of nuclear power plants with low cost storage that has a very highthermal efficiency. As a result of the invention, nuclear reactors willbe able to operate cost-effectively at maximum capacity and will be ableto compete with conventional fossil fueled power plants in theirresponsiveness to load changes over a wide range.

The system and method can utilize a high temperature heat transfermedium, e.g., hot helium (He), and can be used to provide heat for asteam power plant. A steam power plant can, in particular, be used as ithas a high turndown ratio and provides a fast response. Of course, anydevice that can use heat to generate electricity may be substituted. Toincrease its suitability for variable operation, the size of the steampower plant can be enlarged to several times that of the nuclear reactorwithout increasing the size of the nuclear reactor itself.

The invention also provides for a process for operating a nuclearreactor with a capability to store energy and deliver electricity whenneeded. The process comprises removing heat from a core of a nuclearreactor by a circulating liquid or gaseous heat transfer medium. Themethod also includes transferring the heat transfer medium at least oneof directly to a power generating device capable of load following andto a storage system. Additionally, the process includes storing eitherthe heat transfer medium or its heat in a storage system and deliveringthe either the stored heat transfer medium or its heat to thepower-generating device when needed.

The heat transfer medium may be a compressed gas. The compressed gas maybe helium. The heat storage system may comprise a set of tanks or a setof pipes containing or filled with high temperature resistant solidsthrough which hot gas from the nuclear reactor is passed in onedirection heating up the filling and leaving a section of the end cooledsuch that the gas exits the tank at a low temperature to be recycled tothe reactor core leaving a small section cold, and the storage circuitis either switched to another cold tank or stopped. The hot tank mayremain hot as a storage medium until the heat is needed, wherein whenthe heat is needed, a second stream of the same compressed gas is passedin a counter current way to be heated in order to be fed to the powergenerating device and in a closed circuit recycled to the storage andback to the power generating device until only a small section remainshot to insure constant temperature of the hot gas delivered to the powergenerating device.

The heat storage system may comprise a storage vessel configured suchthat heat is absorbed in a way that it spreads through the tank in arelatively sharp front, and preferably less wide than one tenth of thelength of the vessel. The storage vessel may be similar to the design ofa recuperative heat exchanger with the main difference being that in arecuperative heat exchanger the cycles are short and of similar durationand the counter current streams have similar velocities whereas whenused for storage, whereby heating occurs whenever heat is available, andthe heat recovery whenever needed to supply the variable load and thecounter current streams may have totally different velocities. Thepower-generating device may be a steam power plant or a gas turbine. Theheat transfer medium may be a liquid. The liquid may comprise one of amolten salt and a molten metal.

The gas may be compressed and the heat exchanged with a gas of the samecomposition but at lower pressure, which is used in separate circuits todeposit the heat in the storage tank and to recover it when needed tothe power-generating device. The lower pressure may comprise about 3 atmto about 30 atm.

The process may further comprise storing hot liquid in one insulatedtank, transferring it when not needed for power generation to a storagevessel, and when needed using it to provide heat to the power generatingdevice preferably a steam power plant and the cooled liquid to a coldstorage tank and when needed back to the reactor core.

The process may be capable of providing fast load following wheneverneeded by using sufficient storage and a steam power plant is configuredfor a high turndown ratio and fast response. The power-generating devicemay be capable of meeting a maximum variable load expected even when theload is larger than the rated capacity of the nuclear power plant,whereby the nuclear power plant is able to achieve a large capacity forshort times using the stored heat.

The invention also provides for a system for storing heat in a nuclearpower plant, wherein the system comprises at least one tank comprisingsolid media structured and arranged to store heat. The system isstructured and arranged to pass a first fluid through at least one tank,transfer heat from the first fluid to the solid media, store the heat inthe solid media, and transfer the heat from the solid media to a secondfluid.

The first fluid may comprise a compressed gas. The compressed gas maycomprise helium. The second fluid may comprise a compressed gas. Atleast one of the first and second fluids may comprise a compressed gashaving a high pressure. The first fluid may comprise a compressed gasmoving a predetermined velocity. The first fluid may be higher intemperature than the second fluid. The first fluid may pass through atleast one device heated by nuclear fission before entering the at leastone tank. The second fluid may be used to produce steam in a power plantbefore entering the at least one tank. The first fluid may comprise acompressed gas passing through at least one nuclear reactor core. Thesecond fluid may comprise a compressed gas passing through a power plantgenerating electrical power.

The system may further comprise a control system controlling at leastone of: when the first fluid is allowed to pass through the at least onetank and when the second fluid is allowed to pass through the at leastone tank.

The system may further comprise a control system controlling at leastone of: when the first fluid is allowed to pass through the at least onetank, when the first fluid is allowed to bypass the at least one tank,when the second fluid is allowed to pass through the at least one tank,and when the second fluid is allowed to bypass the at least one tank.

The solid media may comprise at least one of: alumina; silica; quartz;ceramic; pebbles made of at least one of alumina, silica, quartz, andceramic; high conductivity and high temperature resistant particles; atleast one packed bed of at least one of particles and pebbles; and atleast one packed bed of solids. The system may be structured andarranged to move at least one of the first and second fluids through theat least one tank with at least one of uniform flow distribution andminimal pressure drops.

The system may further comprise at least one nuclear reactor coreheating the first fluid before the first fluid enters the at least onetank and a steam power plant receiving the heated fluid from the atleast one nuclear reactor core under certain conditions and receivingthe second fluid from the at least one tank under certain otherconditions.

The system may further comprise one or more valves controlling movementof the first and second fluids between the at least one nuclear reactorcore, the at least one tank, and the steam power plant and one or morerecycle compressors pressurizing the first and second fluids.

The first and the second fluid may comprise helium. The first and secondfluids may comprise portions of the same compressed gas flowing in aclosed system, wherein the portions have different temperatures whenentering the at least one tank. The first fluid may comprise a fluidheated by at least one reactor core before entering the at least onetank and the second fluid comprises a fluid exiting a power plant beforeentering the at least one tank. The system may have the following threecycles; a first cycle wherein the first fluid bypasses the at least onetank, flows to a power plant, and returns to at least one reactor core,a second cycle wherein at least a portion of the first fluid flowsthrough the at least one tank and returns to the at least one reactorcore, and a third cycle wherein the second fluid passes through the atleast one tank, flows to a power plant, and returns to the at least onetank.

The invention also provides for a system for producing electrical energycomprising at least one tank comprising solid media structured andarranged to store heat, at least one reactor core heating a first fluidbefore the first fluid enters the at least one tank, and a power plantreceiving the heated fluid from the at least one reactor core undercertain conditions and receiving a second fluid from the at least onetank under certain other conditions. The system is structured andarranged to pass the first fluid through the at least one tank, transferheat from the first fluid to the solid media, store the heat in thesolid media, and transfer the heat from the solid media to the secondfluid.

The system may further comprise one or more valves controlling movementof the first and second fluids between the at least one reactor core,the at least one tank, and the power plant, one or more recyclecompressors pressurizing the first and second fluids, and a controlsystem controlling at least one of: when the first fluid is allowed topass through the at least one tank, when the first fluid is allowed tobypass the at least one tank and pass through the power plant, when thesecond fluid is allowed to pass through the at least one tank, and whenthe second fluid is allowed to bypass the at least one tank and enterthe at least one reactor core.

The system may have three cycles which include: a first cycle whereinthe first fluid bypasses the at least one tank, flows to the powerplant, and returns to the at least one reactor core, a second cyclewherein at least a portion of the first fluid flows through the at leastone tank and returns to the at least one reactor core, and a third cyclewherein the second fluid passes through the at least one tank, flows tothe power plant, and returns to the at least one tank.

The invention also provides for a method of storing heat comprisingmoving a portion of heated fluid from at least one reactor core to atleast one tank comprising solid media structured and arranged to storeheat and transferring the stored heat from the solid media to a fluidthat can be used by a power plant to generate electrical energy.

The heated fluid and the fluid may comprise a compressed gas. Thecompressed gas may comprise helium. The method may further comprisepressurizing at least one of the heated fluid and the fluid to a highpressure.

The invention also provides for a process for providing a nuclearreactor with a capability to store energy and deliver electricity whenneeded, wherein the process comprises removing heat from a core of anuclear reactor by a circulating liquid or gaseous heat transfer medium,transferring the hot heat transfer medium when needed directly to apower generating device capable of load following, and when needed to astorage system, and storing either the heat transfer fluid or its heatin a storage system capable of storing either the heat transfer mediumor its heat and capable of delivering the either the heat transfermedium or its heat to the power-generating device when needed.

The heat transfer medium may be a compressed gas. The compressed gas maybe helium. The heat storage system may comprise a set of tanks or a setof pipes containing or filled with high temperature resistant solidsthrough which hot gas from the nuclear reactor is passed in onedirection heating up the filling and leaving a section of the end cooledsuch that the gas exits the tank at a low temperature to be recycled tothe reactor core leaving a small section cold, and the storage circuitis either switched to another cold tank or stopped. The hot tank mayremain hot as a storage medium until the heat is needed, wherein whenthe heat is needed, a second stream of the same compressed gas is passedin a counter current way to be heated in order to be fed to the powergenerating device and in a closed circuit recycled to the storage andback to the power generating device until only a small section remainshot to insure constant temperature of the hot gas delivered to the powergenerating device.

The heat storage system may comprise a storage vessel configured suchthat heat is absorbed in a way that it spreads through the tank in arelatively sharp front, and preferably less wide than one tenth of thelength of the vessel. The storage vessel may be similar to the design ofa recuperative heat exchanger with the main difference being that in arecuperative heat exchanger the cycles are short and of similar durationand the counter current streams have similar velocities whereas whenused for storage, whereby heating occurs whenever heat is available, andthe heat recovery whenever needed to supply the variable load and thecounter current streams may have totally different velocities. The gasmay be compressed and the heat exchanged with a gas of the samecomposition but at lower pressure, which is used in separate circuits todeposit the heat in the storage tank and to recover it when needed tothe power-generating device. The lower pressure may comprise about 3 atmto about 30 atm. The power-generating device may be a steam power plant,or a gas turbine or, a combination of both. At least one power-producingdevice may comprise a gas turbine is utilized. The heat transfer mediummay be a liquid. The liquid may comprise one of a molten salt and amolten metal.

The process may further comprise storing hot metal in one insulatedtank, transferring it when not needed for power generation to a storagevessel, and when needed using it to provide heat to the power generatingdevice preferably a steam power plant and the cooled liquid to a coldstorage tank and when needed back to the reactor core.

The process may be capable of providing fast load following wheneverneeded by using sufficient storage and a steam power plant configuredfor a high turndown ratio and fast response.

The power-generating device may be capable of meeting a maximum variableload expected even when the load is larger than the rated capacity ofthe nuclear power plant, whereby the nuclear power plant is able toachieve a large capacity for short times using the stored heat.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further described in the detailed descriptionwhich follows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention,in which like reference numerals represent similar parts throughout theseveral views of the drawings, and wherein:

FIG. 1 schematically shows one non-limiting embodiment of a hightemperature nuclear reactor with storage capability.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 provides a schematic of one non-limiting embodiment of theinvention. The system utilizes a nuclear reactor or reactor core RC, adistribution valve system DV, a first helium compressor MCI, a steampower plant SPP, a heat storage system HSS, a helium tank HT, a secondhelium compressor HC2, as well as one or more valves V, and conduits,e.g., pipes, for moving the helium through the system. The solid-line(cycle 1) indicates a flow of He between the reactor core RC,distribution valve DV, the steam power plant SPP, the valve V and thefirst compressor HC1, and then back to the reactor core RC. Thedotted-line (cycle 2) indicates a flow of He between the reactor coreRC, through the distribution valve DV, through the heat storage systemHSS, valve V, and compressor HC1, and then back to the reactor core RC.The dashed-line (cycle 3) indicates a flow of He from the steam powerplant SPP, to the helium tank HT, through the second compressor HC2, tothe heat storage system HSS, and then to the steam power plant SPP.

As is apparent from the FIG. 1, the invention provides for removing andstoring the heat from hot He passing through one or more large storagetanks of the system HSS. The tanks can be filled with a suitable solidfilling, which is resistant to (i.e., which can withstand) hightemperature (e.g., pebbles or particles made from alumina, silica,quartz or ceramics) and preferably have a high heat capacity. Acceptableheat capacities (specific heat) are above 0.15 preferably above 0.2 andmost preferably 0.25 and above. Heat conductivity should be above 2 W/m°K and preferably, above 5 W/m° K. An example would be alumina balls(specific heat 0.27, conductivity 6-20 W/m° K). To minimize both theheating time of a particle and of the total pressure drop, their sizeshould be preferably between 1 to 20 mm and most preferably between 3 to10 mm to get acceptable heating times and pressure drop. While there maybe other materials and other geometric shapes that may be preferable,the selection of appropriate materials and shapes are left to theartisan based upon the instant invention and cost considerations.

In accordance with the features of the invention, the following exampleis provided to further facilitate understanding of the invention. Whenthe full capacity of the nuclear power plant is used to meet the demandfor electricity, all the He from the reactor core RC can be fed directlyto the steam power plant SPP. When the demand for electricity is reducedor, when the plant SPP is to operate from storage HSS, the excess He notrequired in the steam plant SPP is directed or diverted to the storagetanks of the system HSS where its heat is deposited or transferred intothe solid filling. Then, the cool He exits the system HSS and is fedback to the nuclear reactor RC. The storage system HSS is designed toallow the deposited heat to progress as a narrow front along the lengthof the tank(s). The tank(s) should be sufficiently oversized so that thecool end remains relatively cool at the end of the storage cycle. Thesame would apply when the flow is reversed. The hot end of the tank(s)would still stay hot until the end of the heat recovery cycle. Thecapacity of the tank(s) should be sufficient to accommodate the maximumvolume of storage needed.

When the stored heat of the system HSS is used to raise the temperatureof the He (cycle 3), the flow through the system HSS is reversed and thecold He flowing into the system HSS from the second compressor HC2 isfed to the cold end of the system HSS and exits the system HSS hot. Dueto the excellent heat transfer between the gas and the solid heatstoring media, there is practically no energy loss in the heat transfer.The only loss of energy is due to pressure drops through the solid mediabed, and the heat loss through the walls of the system HSS. Both ofthese losses, however, can be minimized by taking these into account indesigning the system. Here, the aim is to make energy storage of thesystem HSS as efficient as possible, and to do so more so than by anyother available method.

When the power requirements of the system exceed normal capacity, allthe He from the reactor core RC can be fed to the steam plant SPP.Additionally, pressurized He in the storage tank(s) of the system HSS isheated and also fed to the steam power plant SPP. This later flowrepresents a recycled counter flow through the storage tank(s) and thenback to the steam plant SPP (cycle 3). The amount of gas in the He cycle3 can be small, i.e., merely sufficient to compensate for the residencetimes in the reactor core RC, the power plant SPP, and the storagetank(s) of the system HSS.

The arrangement described above can be likened to a steam power plantwhich uses stored hot He as a fuel and which stores a supply for one dayof operation (or for whatever period is desired). The steam plant can bedesigned to meet almost any desired delivery schedule as long as thetotal output per day does not exceed the total output of the nuclearreactor. Thus, for intermediate loads, one can operate the plant atdouble the capacity of the nuclear reactor, e.g., twelve hours each day,and store the total output during the night (directing just enough He tokeep the steam power plant hot). In this case, the capacity of the steampower plant would have to be doubled.

The nuclear power plant could also be designed to supply instantaneouslydispatchable electricity with a much larger electricity output than thecapacity of the nuclear reactor itself for a limited period, i.e., basedon demand. For example, by quadrupling the capacity of the steam powerplant, one can supply instantaneously dispatchable electricity up tofour times nominal capacity, as long as the total amount delivered doesnot reach the total capacity of the nuclear reactor for one day. Tooperate in variable mode, or to provide instantaneously availablestandby, however, the output of the steam power plant has to be keptabove 13% of maximum capacity during this period. In this regard, thereactor can be shut down overnight and energy can be stored if enoughheat is supplied to keep it warm.

The invention or aspects thereof can be applied to any other powergenerating device that can convert the energy of the hot heat transfermedium to electricity. It can be assumed, for example, that a grid willbe powered by differently designed reactors; some for base power, (40%of total power requirement of the grid) and others for intermediate loadactivity or load following.

The invention or aspects thereof can also be applied to an HTR in whichhot pressurized He (see Penner, S. S.; Seiser, R.; Schultz, K.; “NuclearEnergy for the Future”, Presented at the Meeting of the Doctors forDisaster Preparedness, Las Vegas Nev., 16-17 Jul. 2005, the disclosureof which is hereby expressly incorporated by reference in its entirety).Furthermore, the invention also contemplates using another pressurizedgas which is expanded in a gas turbine to generate electricity and aftercooling, is re-compressed and fed back to the reactor core. Such plantscan be substituted for the steam power plant in the FIG. 1. However,these other arrangements can limit the applicability of the invention toload following substantially. When used for intermediate loads, combinedcycle gas turbine power plants are shut down at night and weekends andstarted up one hour before needed—so are the gas turbines.

As should be apparent from the FIG. 1, the invention can be used withcombined cycle power plants or with any closed loop gas turbine (see,for example, “Small Nuclear Power Reactors”, UIC Nuclear Issues BriefingPaper #60, June 2006, the disclosure of which is hereby expresslyincorporated by reference in its entirety). These can be used forintermediate power by doubling the capacity of the gas turbine andbypassing it when not in use, storing the heat in the same way asdescribed in the example which follows. In this case, however, fast loadfollowing over large amplitudes is no longer feasible because efficiencydrops severely when operation is below 80% capacity.

The invention can be applied to any nuclear reactor in which the nuclearcore is cooled by a circulating gas or liquid that can be used to heator drive a power-generating device. A liquid heat transfer medium (ofthe type described in, for example, “Small Nuclear Power Reactors”, UICNuclear Issues Briefing Paper 460, June 2006) can also be used the sameway in a tank filled with an appropriate temperature-resistant filling.Alternatively, one storage tank can be used for storing hot liquid andanother for cold liquid. However, a much larger inventory of liquid isrequired when two empty tanks are used, therefore, the system describedin the instant FIG. 1 is normally preferable.

Example

Consider a 250 MW high-temperature nuclear reactor in which the reactorcore RC is cooled by circulating He under pressure. According to theinvention, the hot He is used to raise or produce steam in ahigh-pressure, high-efficiency steam power plant SPP which has a fastresponse, a high turndown ratio and, can operate efficiently at 13% ofcapacity. Then, the gas is recycled cold to the reactor core RC. If themaximum capacity of the steam power plant SPP is increased four-fold to1000 MW, 1000 MW can be delivered for short periods, even though theheat source is sufficient for only an average load of 250 MW. For loadfollowing, the output can be varied over the entire range, 150 to 1000MW. For supplying intermediate power, the steam power plant SPP needs tobe increased to 500 MW, operating 12-13 hours a day. In addition, it isassumed that 12 hours of storage might be optimal.

Assuming also that a steam power plant SPP requires 8000 BTU/KWh, 12times that amount or 96,000 BTU per KW capacity is required to provide12 hours of storage; for the total plant, a storage capability of 24,000MMBTU is required. Given that the heat resistant solid filling of thesystem HSS will have a specific heat C_(P) of 0.25 and that thetemperature drop of the circulating He will be 1400° F., 0.125 tons ofpebbles will be needed per KW installed or 31,200 tons of pebbles forthe total plant, plus an excess of 15% to keep the two end sections atconstant temperature, for a total of 36,000 tons. There are asignificant number of suppliers for ceramic fillings in any desiredshape, suitable alumina balls are made by MarkeTech (for example, gradesP975 and P965). Special ceramic fillers can also be ordered.

Another option would be to use ready made, e.g., 4-foot diameter steelpipes, and have them prepared in a shop to provide 50 to 100 footsections coated in the inside with an insulating heat resistant layer,and designed for easy on-site assembly. The pipes can be providedalready filled with the proper filling material. This is especiallyadvisable if more than one plant is built. In this example, 700 suchpipes, each 100 feet long, would be needed (or, 1200 section, each 60feet long).

In some high temperature nuclear reactors, the pressure of the heliumcan reach 70 to 100 atm. At this pressure, large tanks become expensive.A possible solution is to add a secondary circuit of helium at a lowerpressure (2.0 to 50 atm, and preferably in the range of 2035 atm) andheat exchange it with the primary circuit. The same applies to any othergaseous heat transfer medium used in the primary circuit. Later whenneeded, heat from the storage tank can be transferred to the power plantby the secondary circuit in the same manner as described above. Thisrequires a vessel or tank volume of about 24,000 m³ or 0.1 m³/KW.

It is preferable to use several tanks since a single tank of 24,000 m³is likely too large and not optimal. The number and dimensions of thetanks used in the system HSS will depend on local conditions. Whilevertical tanks placed in the ground are acceptable when conditionspermit, horizontal tanks in which the two end sections are easilyavailable for maintenance may be preferable. Both ends require adistributor and an outlet collection system. There are many provendesigns for distribution and collection developed for catalytic reactorswhich are well-known to those skilled in the art. High L/D ratios arepreferable as they promote an even flow distribution, and a good plugflow.

The example herein provides one possible embodiment. The desired volumeof 20,000 m³ can be achieved by installing 17 tanks placed horizontally,each 8 meters in diameter and 30 meters long. Each tank will provide14,750 KW capacity. The heat flowing through one storage tank is 111million BTU/hr, the temperature drop is 1400° F., and the molar Cp of Heis 5.0 moles. Thus, the total flow of He is 20,570 moles/hr or, 5.7moles/second. In Table 1 we have estimates for a proposed design forthis example using a pressure of 30 atm and a tank with a length of 100feet. It should be noted that the linear velocities are small and thepressure drop and the required re-compression energy for the storage bedis quite small, and for maximum delivery during load following thispressure drop and the compression requirements are acceptable and thestorage efficiency is still very high.

Clearly, the total amount of electricity supplied per day cannot exceed6 GWh/day, i.e., the capacity of the nuclear reactor in the instantexample. With 12-hour storage, the maximum feasible output that can besupplied is 1 GW for 4 hours (of which 1 million KWh would come directlyfrom the reactor RC and 3 million KWh from the storage HSS). Anadditional 2 GWh would have to be dispatched at the rate of 250 MW overa long time period.

The foregoing is an extreme case. In practice, load following up to 500MW for the entire time desired could be provided by one gigawatt outputfor shorter periods. With experience, a practical dispatching schedulethat allows the system to be used for intermediate loads, peak loads andinstantaneously dispatchable energy can be devised, and the system canbe designed accordingly. The proposed system maximizes flexibility byusing multiple tanks and by allowing for an increase in storagecapacity.

It should be apparent that there can be many potential variations inscheduling that fulfill the three constraints of the design: thecapacity of the nuclear reactor, the storage supplied, and the size ofthe steam power plant. With the invention, the response to changes indemand can be as fast as with conventional steam power plants, and thenuclear reactors can always operate steadily at optimum conditions.Detailed cost estimates are not herein discussed, as they stronglydepend on the location, timing and the desired load schedule. However,the following hypothetical example will illustrate the potentialadvantages of the invention.

Consider a 250 MW high-temperature reactor RC cooled with pressurized Heand designed with 12 hour heat storage in the system HSS. Forsimplicity, all costs are based on 1 KW capacity. We assume that thecost of the nuclear reactor complex itself without storage is $2500/KWcapacity of which $350 goes for the steam power plants. To operate inintermediate mode, the capacity of the steam power plant SPP must bedoubled and this adds $350/KW to the cost. When designed for loadfollowing mode, the steam power plant capacity must be increasedfour-fold, raising the base cost by $1050/KW. The cost of heat storageof the system HSS would be the same in each case.

To store heat for 12 KWh, the storage system HSS need per KW capacity is0.125 tons of solid media, which requires a storage vessel with a volumeof 0.1 m³ per KW at a cost of less than $200, if another $100/KW isadded for the cost of the rest of the storage system HSS, the total costof the heat storage is $300 per KW. This brings the total cost to $3,250for the total power plant. To increase the capacity four-fold, another$700 should be added for the steam plant SPP. This brings the total costto $3950/KW of the base plant or about 60% above the cost of thebase-load only cost.

To supply 2 KW intermediate load from the same HTR without storagerequires two 250 MW power plants. The incremental capital cost would be$2,500 compared to $750 for the storage case. Unlike the instantinvention, which includes storage, however, this solution has verylittle load following capability. Where fast load following is required,however, the ability to produce up to 1 GW (as mentioned above) cannotbe matched by any combination of HTRs without storage. Even if this werepossible, the cost would be much higher.

The invention described herein places high-temperature nuclear reactorsat a substantial economic advantage. Today, their market is limitedbecause they are more expensive to build and operate than water-cooledreactors and, their maximum size is small. In addition to increasing thecost-effectiveness of nuclear reactors for base load, the invention alsomakes them economically attractive for supplying the variable demands ofthe grid, which is the major part of the total market for electricity.

It is noted that the forming examples have been provided merely for thepurpose of explanation and are in no way to be construed as limiting ofthe present invention. While the present invention has been describedwith reference to exemplary embodiments, it is understood that the wordswhich have been used herein are words of description and illustration,rather than words of limitation. Changes may be made, within the purviewof the appended claims, as presently stated and as amended, withoutdeparting from the scope and spirit of the present invention in itsaspects. Although the present invention has been described herein withreference to particular means, materials and embodiments, the presentinvention is not intended to be limited to the particulars disclosedherein; rather, the present invention extends to all functionallyequivalent structures, methods and uses, such as are within the scope ofthe appended claims.

TABLE 1 Nuclear Plant Design Parameter Value Nuclear Plant Size (MW) 250Steam Power Plant Size (MW) 1000 Heat Transfer Fluid He Pressure (atm)30 T_(Max) (° F.) 1700 Daily kWh via Storage/kW Installed 12 Power PlantEfficiency (%) 42.6 (1 kWh = 8000 Btu)

TABLE 2 Design of Storage Parameter Value Storage: Number of Vessels 17(Diameter × Length; m × m) (8 × 30) Solid Filling (mm) 10 AverageDiameter Alumina Particles Density (kg/m³) 4000 Bulk Density (kg/m³)2400 Velocity in Storage Tank (m/sec) 0.12 Maximum Velocity in StorageTank 0.48 During Load Following (m/sec) Single Pass Pressure Drops inStorage Tank (atm) 0.024 Maximum Single Pass Pressure Drops in StorageTank 0.45 During Load Following (atm) kWh Compression per kWh Generated0.0006 Maximum kWh Compression per kWh Generated 0.011 During LoadFollowing

What is claimed:
 1. A system for storing heat generated in a nuclearpower plant, the system comprising at least one tank; wherein the atleast one tank contains or is filled with high temperature resistantsolids; the high temperature resistant solids comprising at least one ofparticles and pebbles, the particles and pebbles made of at least one ofalumina, silica, quartz, and ceramic; the at least one tank beingconfigured to receive a first fluid in order to transfer and store heatfrom the first fluid to the high temperature resistant solids; the atleast one tank being also configured to receive a second fluid in orderto transfer the heat from the high temperature resistant solids to thesecond fluid; wherein the at least one tank and the high temperatureresistant solids are configured such that heat is absorbed in the hightemperature resistant solids and that the heat spreads through the atleast one tank in a relatively sharp front; and wherein the relativelysharp front has a width that is less than one tenth of a length of theat least one tank.
 2. The system of claim 1, wherein the first fluidcomprises a compressed gas.
 3. The system of claim 2, wherein thecompressed gas comprises helium.
 4. The system of claim 1, wherein thesecond fluid comprises a compressed gas.
 5. The system of claim 1,wherein the first fluid comprises a compressed gas moving at apredetermined velocity.
 6. The system of claim 1, wherein the firstfluid is higher in temperature than the second fluid.
 7. The system ofclaim 1, wherein the first fluid passes through at least one deviceheated by nuclear fission before entering the at least one tank.
 8. Thesystem of claim 1, wherein the second fluid is used to produce steam ina power plant before entering the at least one tank.
 9. The system ofclaim 1, wherein the first fluid comprises a compressed gas passingthrough at least one nuclear reactor core.
 10. The system of claim 1,wherein the second fluid comprises a compressed gas passing through apower plant generating electrical power.
 11. The system of claim 1,wherein the solid media comprises: at least one packed bed of at leastone of particles and pebbles.
 12. The system of claim 1, wherein thesystem is structured and arranged to move at least one of the first andsecond fluids through the at least one tank with at least one of uniformflow distribution and minimal pressure drops.
 13. The system of claim 1,wherein the first and second fluids comprise portions of the samecompressed gas flowing in a closed system, wherein the portions havedifferent temperatures when entering the at least one tank.
 14. Thesystem of claim 1, wherein the first fluid comprises a fluid heated byat least one reactor core before entering the at least one tank and thesecond fluid comprises a fluid exiting a power plant before entering theat least one tank.
 15. A system for storing heat generated in a nuclearpower plant, the system comprising: at least one tank, wherein said atleast one tank contains or is filled with high temperature resistantsolids; the high temperature resistant solids comprising at least one ofparticles and pebbles made of at least one of alumina, silica, quartzand ceramic; the at least one tank being configured to receive a firstfluid in order to transfer and store heat from the first fluid to thesolid media high temperature resistant solids; the at least one tankbeing also configured to receive a second fluid in order to transfer theheat from the solid media high temperature resistant solids to thesecond fluid; and a control system controlling at least one of: when thefirst fluid is allowed to pass through the at least one tank; and whenthe second fluid is allowed to pass through the at least one tank;wherein the at least one tank and the high temperature resistant solidsare configured such that heat is absorbed in the high temperatureresistant solids and that the heat spreads through the at least one tankin a relatively sharp front; and wherein the relatively sharp front hasa width that is less than one tenth of a length of the at least onetank.
 16. A system for storing heat generated in a nuclear power plant,the system comprising: at least one tank, wherein said at least one tankcontains or is filled with high temperature resistant solids; the hightemperature resistant solids comprising at least one of particles andpebbles, the particles and pebbles made of at least one of alumina,silica, quartz, and ceramic; the at least one tank being configured toreceive a first fluid in order to transfer and store heat from the firstfluid to the solid media high temperature resistant solid; the at leastone tank being also configured to receive a second fluid in order totransfer the heat from the solid media high temperature resistant solidsto the second fluid; and a control system controlling at least one of:when the first fluid is allowed to pass through the at least one tank;when the first fluid is allowed to bypass the at least one tank; whenthe second fluid is allowed to pass through the at least one tank; andwhen the second fluid is allowed to bypass the at least one tank;wherein the at least one tank and the high temperature resistant solidsare configured such that heat is absorbed in the high temperatureresistant solids and that the heat spreads through the at least one tankin a relatively sharp front wherein the relatively sharp front has awidth that is less than one tenth of a length of the at least one tank.17. A system for storing heat generated in a nuclear power plant, thesystem comprising: at least one tank, wherein said at least one tankcontains or is filled with high temperature resistant solids; the atleast one tank being configured to receive a first fluid in order totransfer and store heat from the first fluid to the solid media hightemperature resistant solids; the high temperature resistant solidscomprising at least one of particles and pebbles, the particles andpebbles made of at least one of alumina, silica, quartz, and ceramic;the at least one tank being also configured to receive a second fluid inorder to transfer the heat from the solid media high temperatureresistant solids to the second fluid; at least one nuclear reactor coreheating the first fluid before the first fluid enters the at least onetank; a steam power plant receiving the heated fluid from the at leastone nuclear reactor core under certain conditions and receiving thesecond fluid from the at least one tank under certain other conditions;and one or more valves controlling movement of the first and secondfluids between the at least one nuclear reactor core, the at least onetank, and the steam power plant; and one or more recycle compressorspressurizing the first and second fluids; wherein the at least one tankand the high temperature resistant solids are configured such that heatis absorbed in the high temperature resistant solids and that the heatspreads through the at least one tank in a relatively sharp front;wherein the relatively sharp front has a width that is less than onetenth of a length of the at least one tank.
 18. The system of claim 17,wherein the first and the second fluid comprises helium.
 19. A systemfor storing heat generated in a nuclear power plant, the systemcomprising: at least one tank, wherein said at least one tank containsor is filled with high temperature resistant solids; the hightemperature resistant solids comprising at least one of particles andpebbles, the particles and pebbles made of at least one of alumina,silica, quartz, and ceramic; the at least one tank being configured toreceive a first fluid in order to transfer and store heat from the firstfluid to the solid media high temperature resistant solids; the at leastone tank being also configured to receive a second fluid in order totransfer the heat from the solid media high temperature resistant solidsto the second fluid; wherein the at least one tank and the hightemperature resistant solids are configured such that heat is absorbedin the high temperature resistant solids and that the heat spreadsthrough the at least one tank in a relatively sharp front; wherein therelatively sharp front has a width that is less than one tenth of alength of the at least one tank; wherein the system has the followingthree cycles: a first cycle wherein the first fluid bypasses the atleast one tank, flows to a power plant, and returns to at least onereactor core; a second cycle wherein at least a portion of the firstfluid flows through the at least one tank and returns to the at leastone reactor core; and a third cycle wherein the second fluid passesthrough the at least one tank, flows to the power plant, and returns tothe at least one tank.